Nuclear power plants traditionally have been designed under criteria mandating safe and reliable performance. Such design criteria have confronted investigators with a broad variety of technical challenges which, in the past, have led to stringent and complex active safety systems, in turn leading to increased plant costs, stretched plant construction schedules, and complex licensing requirements. In consequence the power generating industry has experienced a cost escalation rendering alternative power generation approaches more palatable to the power industry.
Over the recent past, however, investigators have addressed these constraints and have looked to a somewhat standardized reactor module which performs within a limited and well defined safety envelope. The result has been the promise for operational attributes providing an inherently safe and passive decay heat removal, factory fabrication, and more economical erection of reactor facilities, the modularization resulting in a smaller reactor size which permits incremental development of reactor sites. The product of this approach has been termed a "Power Reactor Inherently Safe Module" sometimes referred to as PRISM. This modular reactor development involves a fast breeder (FBR) architecture wherein the reactor core is submerged within a reactor vessel containing a liquid metal such as sodium. Also contained within the confining reactor vessel are two intermediate heat exchangers (IHX) carrying out a heat exchange function between the primary liquid sodium of the reactor and a secondary liquid sodium system leading out of the reactor for secondary heat exchange ultimately developing steam derived power.
To circulate primary liquid sodium about the reactor core and the intermediate heat exchanger, a high capacity electromagnetic pump also is submerged within this liquid metal medium. Structured in the manner of a linear motor, the electromagnetic pump has no moving parts, taking advantage of the response capability of the primary sodium to electromagnetic flux influence. Generally, four cartridge-type electromagnetic pumps are employed with this compact pool type reactor and the primary radioactive system is hermetically sealed. When an associated steam turbine, driven from the reactor generated power, is unavailable due to maintenance and the like, decay heat from the reactor is rejected to a condenser. Should the condenser or feed water systems not be available, decay heat is rejected by opening air louvres in a shroud about the steam generator. The natural convection of air over the steam generator shell will cool the reactor module to stand-by conditions in a relatively minimal period of time, for example, two days. Additionally, because of the modular reactor's relatively smaller size, a passive back-up decay heat removal is provided by a radiant reactor vessel auxiliary cooling system (RVACS). Operation is continuous and air movement is by passive convective air current. The core of the reactor is designed to produce only as much heat as is removed. Accordingly, should a downstream heat exchange to steam be terminated for any reason, the reactor system may shut down with no significant safety related concern.
Because of the circulation capacity required of the submerged electromagnetic pumps, a substantial amount of power is called for, i.e. in excess of a megawatt. An electromagnetic pump of this size is a highly inductive component with a lagging power factor of 0.4 to 0.6. For efficiency and compatibility with the solid state power conditioning unit, the power factor must be corrected. One advantageous approach to power factor correction resides in the utilization of a synchronous machine connected in parallel with each of the submerged electromagnetic pumps. Such devices will interject a capacitive term to the electrical power system to effect such correction. For the instant application, an added advantage accrues with the utilization of such synchronous machines. In this regard, in the event of a safety related form of reactor shut-down involving the termination of electrical power, i.e. a scram condition or the like, it will be necessary to provide a continuum of controlled coolant pumping for a limited interval, for example 200 seconds or more, in order to accommodate or remove latent heat from the reactor core. Otherwise, localized boiling with the potential for localized fuel melting may be encountered. By incorporating a flywheel exhibiting sufficient inertial attributes with the synchronous machine, power for this limited interval coastdown becomes readily available. Thus, under abnormal or reactor scram conditions wherein power is suddenly removed from the electromagnetic pumps and associated synchronous machines, the synchronous machines will provide inertially generated electrical power to the pumps such that the thermal power-to-primary coolant flow ratio is appropriately maintained as the reactor commences to cool.
While the electromagnetic pumps employed with the reactor exhibit the advantages of having no moving parts and, inasmuch as they are installed directly within the reactor, no primary fluid is required to leave the vessel. To add assurance to the noted benign performance of the modular reactor power systems, it is desirable to provide a design accommodation to a possibility for certain ground fault conditions which might be developed at the pumps. These linear pumps generally are fashioned as a sequence of electromagnetic coils which are located under an inert atmosphere within a protective shell or sheath fashioned, for instance of stainless steel. Should the insulating architecture of the structure surrounding the coils fail at one position or another, then a single point fault induced current can develop between a given coil or coils and the metallic shell which may cause development of an arc potentially exposing the electromagnetic components of the pump to liquid sodium. In addition to potential secondary reactions, this liquid metal coolant may be carrying nuclear particles from a failed fuel pin within the reactor. It therefore becomes incumbent to provide a design by which such an excursion cannot occur and which addresses the overall design critiera for the modular reactors calling for an inherency of reliability and safety. Accordingly, should such a fault condition occur within one of the pumps, then the fault should be contained in a manner wherein safe operation is maintained and the reactor may continue to perform in a safe manner by virtue of its redundant structuring. Further, the occasion of such a fault, albeit controlled, should be the subject of astute monitoring and analysis on the part of plant operating personnel. Such control over ground faults further should be applied with respect to the noted synchronous machine, as well as to the housings of associated, isolated power supplies and conditioners.